In general, an optical head device for an optical disk often employs a single lens having an aspheric surface as an objective lens for recording or reproducing information by focusing light beams into a diffraction limited point image on an information medium. However, in recent years, it is increasingly required to reproduce data compatibly with optical disks each having a different thickness. For example, it is required to read a CD (compact disk) or a CD-ROM having a disk thickness of 1.2 mm and a DVD (digital versatile disk) or DVD-ROM having a disk thickness of 0.6 mm by a single optical head. In this case, one method is to use two objective lenses, one lens corresponding to and one lens corresponding to the CD or CD-ROM having a disk thickness of 1.2 mm and one lens corresponding to the DVD or DVD-ROM having a disk thickness of 0.6 mm. However, from the viewpoint of simplifying an optical system, it is advantageous to use a single lens that is applicable for two kinds of optical disks each having a different thickness. In other words, in the optical head using two objective lenses, since a mechanism for switching lenses is needed, the configuration of the optical head becomes complicated, thus making it difficult to achieve a small size and low cost. Furthermore, it is desirable that the objective lens be as simple as possible.
For example, in JP8 (1996)-334690A and JP9 (1997)-184975A, a so-called two-zone divided type lens is disclosed as an objective lens in order to attain the above-mentioned object. In this configuration, a lens is divided into two regions, in which an inner circumference portion is designed to be suitable for use with a CD or CD-ROM, and an outer circumference region is designed to be suitable for use with a DVD or DVD-ROM. Hereinafter, a conventional objective lens will be described with reference to FIG. 28. FIG. 28 is a view of an arrangement showing a relationship between an objective lens and an optical disk in the prior art.
FIG. 28(a) is a view showing an optical path of a two-sided aspheric objective lens 33 when a light beam is focused onto an optical disk 32 having a thickness of 0.6 mm. The surface of the objective lens at the side of the light source includes an outer circumference region 34 and an inner circumference region 35. In the outer circumference region 34, the spherical aberration is corrected with respect to the optical disk 32 having a thickness of 0.6 mm. On the other hand, in the inner circumference region 35, the spherical aberration is corrected with respect to the optical disk having a thickness of 0.9 mm. The boundary between the inner circumference region 35 and the outer circumference region 34 is determined by the NA that is necessary to reproduce data of the disk having a thickness of 1.2 mm for a light source of a wavelength of 655 nm. For example, when an optical disk having a thickness of 1.2 mm is reproduced at a wavelength of 780 nm and a NA of 0.45, the NA of the light source of 655 nm is about 0.37. The inner circumference region 35 has a spherical aberration with respect to the optical disk having a thickness of 0.6 mm. Herein, the total aberration is much smaller than 0.07 λ (which is called a diffraction limit) and falls in a sufficient aberration for reproducing the optical disk of a thickness of 0.6 mm.
FIG. 28(b) is a view showing an optical path when a light beam is focused onto an optical disk 36 having a thickness of 1.2 mm by using the same objective lens 33. Since the inner circumference region 35 of the same objective lens 33 is optimized with respect to an optical disk having a thickness of 0.9 mm, the aberration is small for the optical disk 36 having thickness of 1.2 mm. However, since the outer circumference region 34 is optimized with respect to an optical disk 32 having a thickness of 0.6 mm, the aberration is large for the optical disk 36 having a thickness of 1.2 mm, so that it does not contribute to the light focusing. Therefore, the outer circumference region 34 also functions as an aperture.
When there is one light source, under the above-mentioned conditions, sufficient performance with respect to two kinds of optical disks each having a thickness of 0.6 mm and 1.2 mm respectively can be attained. However, when it is necessary to reproduce the optical disk having a thickness of 1.2 mm by using the light source of 780 nm like CD-R, the wavelength becomes longer. Consequently, it is necessary to increase the NA relatively, so that the aberration with respect to the optical disk having a thickness of 0.6 mm becomes larger, thus deteriorating the focusing property.
Furthermore, not shown in FIG. 28, a mold for molding an objective lens with a difference in level between an inner circumference portion and an outer circumference portion has been produced by cutting with the use of a diamond cutting tool in order to precisely work a portion having the difference in level.
Furthermore, in a profile measuring device for measuring and evaluating the worked mold, a rotationally symmetric aspheric surface is used as design profile data which is reference data for evaluating measurement values. When a zone-divided type objective lens is measured, the inner circumference portion and the outer circumference portion are measured separately, or a fitting of the profiles of the inner circumference portion and the outer circumference portion is carried out by the use of a higher order aspheric coefficient of about 20-th order to express the profile and the profile is compared with the measurement values.
When the above-mentioned zone-divided type objective lens has the portion having the difference in level between an aspheric surface of the inner circumference portion and an aspheric surface of the outer circumference portion, an ideal design is to provide a surface forming the difference in level approximately parallel to the optical axis. However, in actual working, it is necessary to weaken the surface forming the difference in level to such an extent that the portion having the difference in level can be worked. JP 9 (1997)-184975A discloses the profile in which the portions having the difference in level are smoothly connected. However, it is a profile in which an aspheric surface of the inner circumference portion and an aspheric surface of the outer circumference portion are weakened approximately uniformly. Such a profile can be obtained only by cutting with the use of a cutting tool. Herein, in cutting with the use of the cutting tool, there is a limitation on the type of metal material that can be worked. Unless a relatively soft metal having excellent cutting properties is used, a sufficient working accuracy cannot be obtained. On the other hand, for press molding the lens from a glass material, it is necessary to mold the lens at high temperature and high pressure. It is desirable that a metal material having high hardness, such as a cemented carbide (a sintered metal including WC as a main component) is used. However, the cemented carbide cannot be worked by cutting with the use of cutting tool. In other words, the zone-divided type objective lenses, which have been proposed to date, have a profile for plastic molding. Therefore, most of the conventional objective lenses molded by the use of glass materials have a difficulty in working a mold.
Furthermore, the refractive index of the lens using a plastic material is changed greatly due to a change in temperature. For example, in an optical head device that is necessary to secure operation in a wide range of temperatures, for example, for vehicle apparatus, in general, glass lens is used. However, as mentioned above, a zone divided type DVD/CD compatible lens is designed based on plastic molding. Thus, the temperature characteristics are extremely deteriorated.
Furthermore, in a profile measuring device for measuring and evaluating a mold for molding a lens, the measured profile and the preliminary stored design profile are compared with each other and a working error is calculated. However, a design profile that can be input is only a profile such as a rotationally symmetric aspheric surface. In the method for measuring the zone divided type objective lens by separating the inner circumference portion and the outer circumference portion, since the entire area of the lens is not measured at one time, it is impossible to know the error in profile exactly. Furthermore, in the method of measuring the profile of the entire lens by expressing the profile of the entire lens by fitting with the use of a higher order aspheric coefficient and using this aspheric profile as a design profile, the fitting error occurs around the portion having the difference in level, and thus sufficient measuring accuracy cannot be obtained.